The present disclosure is directed to a compound suspension linkage and in particular such a system for installation on a well logging tool for use in well boreholes to position an oil well survey instrument on a extendable pad for contact against the side wall of the borehole. Caliper measurements can be obtained also. The present apparatus is used with a logging tool which requires contact against the side wall. It is necessary to accommodate a number of factors in accomplishing side wall logging. First of all, the pad must be positioned against the side wall for skidding movement as the well tool is retrieved on the logging cable in the borehole to the sonde. Many factors come into play, all having a bearing on the lateral load applied by the sensor pad to the borehole. For instance, variations in hole diameter are typically encountered. It is necessary therefore to provide a suspension system which deflects to position a sensor pad at the side wall which can flex or rotate relative to the tool body as variations in hole diameter are encountered. The position of the sonde may be precisely on the center line or may be off to the side. Another factor which is important is the provision of a relatively constant lateral load. The force should be the same without regard to the lateral deflection of the sensor pad. The sensor pad may momentarily slide past hard rock formations or may slide past unconsolidated sands. In all instances, the side wall typically will be coated with drilling fluid which forms a mud cake. The drilling fluid is typically formed of barites, and will vary in thickness. It is intended to coat the side wall and form a sealing surface sometimes known as a mud cake. The sensor pad must slide over this mud cake and yet maintain an approximately constant lateral load against the side wall of the borehole. The present apparatus is intended for use in wells that are vertical, or nearly so. In addition, it is successful in highly deviated wells where the tool is inclined at a severe angle relative to gravity. In that instance, the compound suspension linkage of the present disclosure may be required to support a substantial load. Even still, the load on the sensor pad must be variable so that it is held approximately constant in load. Variations in sensor pad loading might produce a different in the coupling of the sensor pads to the formations adjacent to the borehole and might have an undesirable impact on the quality of data. The present disclosure sets forth a system whereby the sensor pad is extended to the side of a sonde from a retracted or streamlined position. In the streamlined state, the sensor pad is fully retracted into a cavity out of harms way and the sonde is therefore able to enter the most narrow of boreholes. Deployment causes the sensor pad to position laterally to the side with controlled loading to maintain contact against the borehole. Since this must occur with a controlled force whether the well is vertical or highly deviated, the present apparatus must provide a variable force to accommodate the changes in tool orientation. It is therefore desirable to provide flexure with a spring loading system for urging the sensor pad against the borehole.
An important feature of the present disclosure is the ability to also measure the caliper of the hole. This depends on the extension of the suspension linkage and a sensor is included for measurement of that. When the tool is fully retracted, a sensor measurement is formed which indicates this retraction. When the linkage is extended, such movement is noted, and various points between are likewise indicated with accurate measurement.
There are many alternate structures known in the art and one is indicated by U.S. Pat. No. 3,167,707. In this disclosure, and in particular FIGS. 1 and 2d, a type of double pivot pantogram is set forth where one leg of the pantogram terminates in a tab connected with a motor device for applying rotating movement to one side of the pantogram, hence causing the entire structure to move. This system incorporates the sensor pad as one side of pantogram. A similar pantogram is also shown in U.S. Pat. No. 4,614,250 where FIG. 4 thereof incorporates a spring attached to the center of the sensor pad and provides a form of loading. This is achieved in a dip meter construction. These and other structures are believed to be deficient in the particulars mentioned and are devoid of features enabling flexure of the sensor pad as it rides up and down with spring loading to assure sensor pad relative rotation for smoothly sliding over irregularities in borehole profile.
The present structure is summarized as providing a compound suspension linkage construction. Rather than position the sensor pad as part of the remote pantogram side, it is affixed at one end and is mounted on a pivot which enables rotative movement of the sensor pad with respect to the central axis of the sonde which supports the structure. Moreover, a leaf spring connects to the back of the sensor pad for providing a controlled load against the borehole. The sensor pad is thus pivotally mounted at one end and is spring loaded at the central portions thereof. The sensor pad is positioned in its unloaded posture so that it is approximately parallel to the central axis of the sonde. This enables it to move outwardly through greater distances. By means of appropriate push rods with adjustable pivots, the compound linkage can extend outwardly while maintaining the sensor pad approximately parallel to the axis of the sonde. Retraction positions the sensor pad in a receptacle so that it is fully recessed and out of the way. When extended, the survey pad is lifted out of the receptacle and deployed, and later moves back into the receptacle subject to control as described.